Concrete has proven to be the preferred material for the construction of roadways in many locales. In such applications, however, concrete invariably develops cracks throughout the length of the concrete structure caused by the curing process, load induced stress, weather conditions, and other causes, so that the life cycle and the ride quality of the concrete road can become severely reduced unless some means is used to prevent the concrete from separating along these crack lines. One method commonly used for this purpose, known as continuously reinforced concrete paving (“CRCP”), incorporates deformed steel concrete reinforcing rods within the entire length of the concrete structure.
Deformed steel concrete reinforcing bar (“rebar”) is used almost exclusively to provide structural reinforcement to concrete structures and is produced in accordance with national standards. It is formed using ferrous scrap metal as the principal raw material. The scrap metal is melted in an electric arc furnace, further processed in a ladle arc-refining unit, and the molten steel is then continuously cast into rectangular billets of steel that are cut to length. The billets are then rolled into various sizes of rebar, which is cut to various lengths depending on the customers' requirements. Deformed rebar is rolled with deformations on the bar, which provides gripping power so that concrete adheres to the bar, and the bar, thereby, provides reinforcing value to the concrete. The deformations include a horizontal rib where hot steel is squeezed out between rollers and various patterns of semicircular ribs spanning the longitudinal ribs, such ribs being referred to herein sometimes as raised elements on a transverse bar. The deformations must conform to certain requirements set out in the national standards. Bar designation numbers correspond to diameter and grade. National standards identify two grades of rebar, e.g. regular or “R” and weldable or “W”. R grades are intended for general applications, while W grades are used where welding, bending or ductility is of special concern. National standards also identify yield strength levels.
Rebar used to reinforce concrete when paving highways is laid out and connected in a rectilinear grid structure called a rebar mat. Rebar that is designed to extend across the width of the highway lane is called “transverse” rebar, and rebar that is designed to extend along the length of a highway lane is called “longitudinal” rebar. Sometimes “bar” is used herein more for rebar to be laid in a direction transverse to the run of the road bed (“transverse bar”) and “rod” more for rebar to be laid parallel to the run of the road bed (“longitudinal rod”). However, when speaking herein of rebar, the term “bar” is used interchangeably with “rod”. No distinction is meant by the use of one term or the other. When cracks develop in concrete, the rebar mat performs the functions of holding the cracks tight, facilitating load transfer across the cracks, and providing stiffness by restraining end movement, thus preventing separation of the concrete and failure of the paving slab.
When designing highway specifications considering the type of road and local environmental conditions, civil engineers determine the optimum spacing of longitudinal rods laid out along transverse bars, the optimal spacing separating transverse bars, and the optimum height of a grid of transverse bars and longitudinal rods above the road bed within the concrete. In order for transverse and longitudinal reinforcing steel to effectively perform their function, the reinforcing steel must remain at its designed location within the concrete slab during and after concrete placement. This requires elevating the rebar mat to the designed height above the road base before the pour, and preventing the reinforcing bars of the mat from moving during the pour. Maintaining the correct height, spacing and shape of the rebar mat during the pouring process is critical to the performance of the completed pavement. A support system for rebar mats that can be imbedded in the concrete slab during the pour is an essential step to the construction of a continuously reinforced concrete roadway. Since the process of imbedding the support system in the concrete consumes the support, the support must be relatively inexpensive.
Most major concrete highway construction is done nowadays with a slip form paving machine that slips the concrete forms alone the sides of the roadway as the machine moves longitudinally along the new roadway structure being paved. The concrete mix used in slip form paving must be relatively dry so that while supported by the forms the concrete will set up sufficiently to hold its shape after the forms slip forward off the structure. This type of concrete mix has a dough-like consistency and is highly viscous and stiff. Great forces have to be applied to the concrete mix by mechanical spreaders and paving vibrators to push and work the stiff mix into place. These forces are in turn transferred by the mass of the mix onto the rebar mat. A support system for holding the rebar mat in its design location for the job to conform to engineering specifications must function to prevent the rebar in the mat from being displaced by the forces impressed on the mat during placement and working of the concrete mix.
One traditional method for erecting a rebar mat for CRCP roadway construction is to assemble the rebar mat in place, and then prop the mat to the design height above the road base. Using this method, the transverse rebar steel is laid out on the ground at the specified intervals. Some longitudinal rebar is then laid out on top of the transverse bars, and these transverse and longitudinal bars are wire tied together to form a template for the completed mat. The template rebar is then lifted up, and supports for the template, called “chairs”, are placed under the transverse bars at the ends of the bars and at locations between the ends of the bars in a number sufficient to support the weight of the steel mat when it is completed. Remaining longitudinal rods are them placed on the elevated template at the design spacing and wire tied to the transverse bars to complete the assembly of the rebar mat.
However, this traditional system of forming and preparing a rebar grid for a concrete pour has several deficiencies. First, with a conventional chair support that props up the rebar mats, the steel simply “sits” on the support, hence origination of the term “chair”. As the concrete mix is forced under the mat, the chairs are frequently pushed (“rotated”) out from under the mat, causing inadequate support for sections of the mat or in extreme cases, allowing the entire mat to fall. Some means are needed to prevent the entire steel mat from moving or “walking” forward or being “racked” out of square as the stiff concrete mixture is worked into place. The traditional solution involves driving a metal stake into the ground at regular intervals to hold the mat in place. These metal stakes, however, can produce premature corrosion of the rebar steel by introducing a rust path to the mat steel and by providing a conductor for cathodic corrosion. Excessive corrosion of the rebar mat produces internal expansion forces that cause the entire concrete slab to crack and fail.
A recent improvement over the use of conventional paving chairs is described in U.S. Pat. Nos. 5,893,252 and 6,112,494, and employs a bar support device fixing transverse and longitudinal steel bars at their intersection with a locking cap that secures the device to the mat and at the same time holds the steel bars together. This system eliminates the need to wire tie the intersections of transverse and longitudinal bars where the support is placed and avoids the problems associated with use of conventional chairs described above. As a result, this chair improvement system has virtually replaced use of conventional roadway construction paving chairs in jurisdictions everywhere state highway departments allow use of wire tied rebar mats.
Some states not in sun-belt winter climates of U.S. do not permit the use of wire tied rebar mats. This is because in these states, winter road deicing considerations require that rebar steel must be coated with an epoxy resin to isolate the steel within a corrosion free environment. Epoxy coated steel has an extremely slippery surface compared to uncoated rough rebar, and in assembling rebar mats made of epoxy coated steel it is economically difficult to achieve a tight connection of the transverse and longitudinal bars by wire tying them together under the wage structure environments typically found on road and highway construction projects in these states. In view of this practical and economic difficulty, the states that require epoxy coated rebar typically specify erection of epoxied rebar mats using prefabricated, welded and epoxied transverse bar assemblies (“TBA's”). TBA's are constructed by spot welding a plurality of spaced open ended U-shaped clips to reinforcing steel bars that are to be placed on the road bed in the transverse direction. The reinforcing steel bars also have steel legs welded to the underside to support the bar at the desired height off the paving sub-base. After the clips and legs are welded to the rebar, the welded assembly is epoxy coated. The TBA's are then transported to the highway paving site, where workers lay them transversely to the run of the road bed to be paved, then place longitudinal bars in each U-shaped clip on top of the TBA's. The TBA legs are supposed to support the longitudinal bars at the designed height or clearance above the road base, and the U-shaped clips are supposed to locate the longitudinal rods at the engineered spacing along the span of the transverse bar and maintain that spacing during the concrete pour.
Epoxy coating of the TBA's has proven problematical. The irregular shape of the weld joints where the U-shaped clips and the bases are affixed to the transverse bar makes achieving a complete epoxy seal of this part a practical impossibility. Further, welding the U-shaped clips and legs to the rebar steel presents a problem at the pour site unique to TBA's. Rebar steel typically has a high carbon content, making it difficult to obtain a solid welded joint, and this is exacerbated with spot welded U-shaped clips, because these have a small steel-to-steel contact area for the weld. At the pour site, laborers laying out the grid for the reinforcing mat drop the longitudinal bars onto the U-shaped clips, sometimes with enough drop force to break the weld, causing the U-shaped clip to fall off the TBA. The site where the clip is missing allows the unrestrained longitudinal bar to displace laterally at that position. Moreover, breaking off the U-shaped clip exposes bare metal to the potentially corrosive environment. The spot weld holding the TBA legs to the TBA rebar is subject to much the same weld weakness as the clips, and the legs can snap-off.
The TBA approach to rebar layout and erection has the same problems of rebar mat instability and potential for corrosion that occurs where rebar mats are wire tied and conventional rebar support chairs are used. The clips on the TBA do not fix the longitudinal bars to the transverse bar. The longitudinal bars merely sit in the clips. Thus the design of the TBA affords no means for preventing the forces of concrete mix placement from pushing or “walking” a TBA, and consequently, does not assure that the engineered spacing between transverse bars is maintained during paving. Loss of specified transverse bar spacing creates the possibility of excess longitudinal rod sagging from lack of design interval support. Further the non-locking design of the TBA affords no means for preventing the forces of concrete mix placement from angularly “racking” a TBA out of square into a shape that has less effective reinforcing capacity and that misaligns longitudinal rod ends from positions designed for attachment to the next adjoining section of rebar mat. Still more, the design of the TBA allows the legs of a TBA to be pushed or rotated out from under the longitudinal steel in the mat, leaving the longitudinal steel not only vertically unsupported where they are supposed to be supported, but also, due to the rotation of the support out from under the longitudinal rebar, allows the clips welded to the transverse bar to rotate out from under the longitudinal rods, releasing them from restricted lateral movement. In order to mitigate this potential for “walking” or “racking” of the mat or “rotation” of the TBA out from under the mat, paving contractors frequently employ the same staking process described above with conventional paving chairs used with wire tied rebar mats. This produces the same potential for cathodic corrosion and a rust path from outside the concrete structure to the rebar mat within the concrete for attack where any steel is exposed by incomplete coating or by broken and knocked off clips.
Any cost savings on Field labor realized by a paving contractor using TBA's rather than conventional paving chairs is more than offset by the cost of the fabricated TBA's. Large scale production spot welding of the U-clips and triangular bases to transverse reinforcing steel bars can be accomplished economically only by deployment of sophisticated robotics welding equipment, at a very large initial capital cost. Moreover, epoxy coating of fabricated TBA's requires a special method and coating chamber not required for epoxy coating unwelded reinforcing steel bars. As a result of the manufacturing costs, the total cost of building a CRCP roadway with TBA's may actually exceed the higher labor costs associated with wire tying if paving chairs were used, yet provide little practical performance improvements during the concrete placement.